1. Field of the Invention
The present invention relates to a technology for correcting a position of a pixel formed on an image carrier by a laser beam in a main scanning direction.
2. Description of the Related Art
An image forming apparatus such as a laser printer, a laser facsimile, and a digital copier includes an optical writing device or an optical scanning device that has an optical deflector (polygon mirror) which deflects and reflects a light flux modulated according to an image signal, and has an image carrier to which an image is written by scanning the light flux deflected by the optical deflector. As a unit for generating the light flux, a laser light source such as a semiconductor laser is generally used. A color image forming apparatus that forms a color image is configured to modulate laser beams by image signals for colors of, for example, yellow (Y), cyan (C), black (K), and magenta (M), respectively, and form images for Y, C, K, and M on four respective scanned surfaces. These color images are transferred to a transferred unit such as a recording paper in a superimposed manner, to form a color image.
In this image forming apparatus, when a plastic lens is particularly used as an fθ lens for converting a laser beam deflected at an equiangular velocity by the optical deflector to that at a constant linear velocity, a linear expansion coefficient of plastic is comparatively large. Therefore, displacement of a beam spot cannot be negligible. More specifically, the displacement occurs caused by changes in shape and reflection coefficient of the plastic lens due to a change in environment temperature and a change in temperature in the apparatus. Furthermore, there may be a case where the reflection coefficient is changed in each beam due to changes or variations in wavelengths of laser beams and displacement in a beam spot cannot thereby be neglected. In this case, a scanned position on the image surface of the image carrier is changed, and this causes a magnification error in the main scanning direction to occur and a high-quality image not to be obtained.
To resolve the problems, Japanese Patent Application Laid-Open No. 2003-185953 discloses an image forming apparatus as follows. In the image forming apparatus, a laser beam is detected by laser beam detectors provided in a leading edge side of the laser beam (scanning start side) and a trailing edge side thereof (scanning end side) outside an effective writing area in the main scanning direction. Then, a time from detection of the laser beam by the laser beam detector in the leading edge side to detection thereof by the laser beam detector in the trailing edge side is measured, and frequency and phase of a clock for writing a pixel forming an image signal are changed according to the result of measurement, and a scanning magnification of each laser beam is corrected, to thereby correct a magnification error and a color shift in an image occurring due to the change in environment temperature and the change in temperature in the apparatus.
FIG. 30 is a diagram of a laser-beam scanning device in the image forming apparatus. This laser-beam scanning device includes laser diodes (LDs) 109Y, 109C, 109K, and 109M for colors of Y, C, K, and M, respectively; and LD driver boards 112Y, 112C, 112K, and 112M for turning on the LDs 109Y, 109C, 109K, and 109M in synchronization with the respective image signals for Y, C, K, and M. This scanning device also includes collimate lenses 113Y, 113C, 113K, and 113M for changing the laser beams emitted from the LDs 109Y, 109C, 109K, and 109M to parallel pencils, respectively; and mirrors 114C and 114K for reflecting the laser beams having been changed to the parallel pencils by the collimate lenses 113C and 113K and changing each direction of the laser beams. This scanning device further includes a polygon mirror 102 for deflecting, at an equiangular velocity, the laser beams passing through the collimate lenses 113Y and 113M and the laser beams reflected by the mirrors 114C and 114K; an fθ lens 103-1 for performing equiangular velocity/constant linear velocity conversion on the laser beams of Y and C deflected by the polygon mirror 102; and an fθ lens 103-2 for performing equiangular velocity/constant linear velocity conversion on the laser beams of K and M deflected thereby. This scanning device further includes photosensitive elements (image carriers) 104Y, 104C, 104K, and 104M for Y, C, K, and M, respectively; return mirrors 115Y, 115C, 115K, and 115M for reflecting the laser beams having passed through the fθ lenses 103-1 and 103-2 to the directions of the photosensitive elements 104Y, 104C, 104K, and 104M, respectively; and optical sensors 105Y, 105C, 105K, 105M, 106Y, 106C, 106K, and 106M which are laser beam detectors for detecting leading edges and trailing edges, in the main scanning direction, of the laser beams reflected by the return mirrors 115Y, 115C, 115K, and 115M, respectively. Each of these optical sensors is formed with a photo integrated circuit (IC) integrated with a photodiode, a phototransistor, or these peripheral circuits.
FIG. 31 is block diagram of an electrical configuration of the laser-beam scanning device of FIG. 30. The optical sensors 105Y, 105C, 105M, and 105K and the optical sensors 106Y, 106C, 106M, and 106K detect each leading edge and each trailing edge of the laser beams for Y, C, M, and K, respectively, by scanning the laser beams, and generate laser-beam leading-edge detection signals (hereinafter, “leading-edge detection signals”) DETP1Y, DETP1C, DETP1M, and DETP1K, and also generate laser-beam trailing-edge detection signals (hereinafter, “trailing-edge detection signals”) DETP2Y, DETP2C, DETP2M, and DETP2K, respectively. These laser-beam detection signals are sent to time-difference measuring units 107Y, 107C, 107M, and 107K, respectively. The time-difference measuring units 107Y, 107C, 107M, and 107K have calculation functions of measuring and averaging time differences between the leading-edge detection signals and the trailing-edge detection signals, respectively. More specifically, the leading-edge detection signals DETP1Y, DETP1C, DETP1M, and DETP1K are sent from the optical sensors 105Y, 105C, 105M, and 105K respectively, and the trailing-edge detection signals DETP2Y, DETP2C, DETP2M, and DETP2K are sent from the optical sensors 106Y, 106C, 106M, and 106K, respectively. The time-difference measuring units 107Y, 107C, 107M, and 107K perform the measurement and calculation according to a setting timing signal from a control unit (central processing unit (CPU)), which is not shown, and send the results of the measurement and calculation to magnification correction controllers 110Y, 110C, 110M, and 110K, respectively.
Each of the magnification correction controllers 110Y, 110C, 110M, and 110K includes a storage unit that stores a set write clock frequency and an initial set value and/or a current set value of a phase control value sent from the control unit (CPU). Each of the magnification correction controllers 110Y, 110C, 110M, and 110K also includes a function of calculating optimal write clock frequency and phase control value according to each of the results of the measurement and calculation in the time-difference measuring units 107Y, 107C, 107M, and 107K. More specifically, the calculation is performed by using a change in image magnification in the main scanning direction due to a change in frequency of the write clock, and by using a change in image magnification in the main scanning direction due to a change in phase of the write clock by a unit smaller than a unit of adjustment (one main scanning) by which a frequency of the write clock is adjusted. Each of the magnification correction controllers 110Y, 110C, 110M, and 110K further includes a function of fixing a write clock frequency and calculating an optimal phase control value according to the results of the measurement and calculation in the time-difference measuring units 107Y, 107C, 107M, and 107K, includes a function of comparing the phase control value with a reference value set in the control unit (CPU), and sends a control signal indicating setting of a write clock and implementation of phase control based on the setting in the control unit (CPU), to each of write-clock generator circuits 108Y, 108C, 108M, and 108K.
The write-clock generator circuits 108Y, 108C, 108M, and 108K include frequency modulators and phase controllers, respectively. Each of the frequency modulators generates a phase-locked loop (PLL) oscillation clock having a frequency n-times of that of write clocks PCLKY, PCLKC, PCLKM, and PCLKK for writing image signals for Y, C, M, and K, in response to reception of clocks from an oscillator (not shown). Each of the phase controllers includes a function of dividing the PLL oscillation clock by n in synchronization with each of the leading-edge detection signals DETP1Y, DETP1C, DETP1M, and DETP1K which are synchronization detection signals, and generating each of the write clocks PCLKY, PCLKC, PCLKM, and PCLKK in synchronization with each of the leading-edge detection signals DETP1Y, DETP1C, DETP1M, and DETP1K. Each of the phase controllers also includes a function of shifting a write clock period pixel by pixel by adding or subtracting an integral multiple of a half period of the PLL oscillation clock to or from each specific period of the write clocks PCLKY, PCLKC, PCLKM, and PCLKK. The write-clock generator circuits 108Y, 108C, 108M, and 108K generate the write clocks PCLKY, PCLKC, PCLKM, and PCLKK and perform phase control under control by the magnification correction controllers 110Y, 110C, 110M, and 110K, respectively.
Each of the write clocks PCLKY, PCLKC, PCLKM, and PCLKK is subjected to image magnification correction in the main scanning direction by changing frequency and phase in the respective write-clock generator circuits 108Y, 108C, 108M, and 108K, and is sent to respective LD modulators 101Y, 101C, 101M, and 101K, which are light-beam generator drivers. The LD modulators 101Y, 101C, 101M, and 101K control turning on of the LDs 109Y, 109C, 109M, and 109K according to image signals in synchronization with the write clocks PCLKY, PCLKC, PCLKM, and PCLKK from the write-clock generator circuits 108Y, 108C, 108M, and 108K, respectively. Therefore, laser beams modulated according to the image signals are emitted from the LDs 109Y, 109C, 109M, and 109K, and the laser beams emitted are deflected by the polygon mirror 102 and pass through an fθ lens 103, to scan the photosensitive elements 104Y, 104C, 104M, and 104K, respectively.
In the conventional image forming apparatus, however, to resolve nonuniform heat affecting an optical system that includes LDs, lenses, mirrors, and a housing (hereinafter, “optical system”), a time-difference measuring unit that controls magnification correction is provided in all the stages for Y, C, M, and K. This causes the number of components to increase. Furthermore, there may be a case where a time-difference measuring unit cannot acquire accurate time difference data because of failure in any of corresponding optical sensors that detects the leading edge or the trailing edge of a laser beam scanning across an image carrier (photosensitive element) for a certain color. In this case, the time difference data cannot be obtained from the relevant time-difference measuring unit until the optical sensor is replaced, and hence, magnification correction for the certain color cannot be performed.